Apparatus and method for non-invasively measuring cardiac output

ABSTRACT

Apparatus and methods for non-invasively determining the cardiac output or pulmonary capillary blood flow of a patient using partial re-breathing techniques. The apparatus includes a substantially instantaneously adjustable deadspace volume for accommodating differences in sizes or breathing capacities of various patients. The apparatus may be constructed of inexpensive elements, including one or more two-way valves, which render the apparatus very simple to use and inexpensive so that the unit may be employed as a disposable product. The method of the invention includes estimating the cardiac output or pulmonary capillary blood flow of a patient based on partial pressure of alveolar CO 2 , rather than on the partial pressure of end tidal CO 2 , as previously practiced. A computer program for calculating the cardiac output or pulmonary capillary blood flow of a patient is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of application Ser. No.09/777,629, filed Feb. 6, 2001, pending, which is a continuation ofapplication Ser. No. 09/262,510, filed Mar. 2, 1999, now U.S. Pat. No.6,227,196, issued May 8, 2001, which is a continuation-in-part ofapplication Ser. No. 08/770,138, filed Dec. 19, 1996.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to non-invasive means of determiningcardiac output or pulmonary capillary blood flow in patients and, morespecifically, to partial re-breathing systems and methods fordetermining cardiac output or pulmonary capillary blood flow inpatients.

[0004] 2. Background of Related Art

[0005] It is important in many medical procedures to determine ormonitor the cardiac output or the pulmonary capillary blood flow of apatient. Cardiac output is the volume of blood pumped by the heart overa given period of time. Pulmonary capillary blood flow is the volume ofblood that participates in gas exchange in the lungs. Techniques areknown and used in the art which employ the use of catheters insertedinto blood vessels at certain points (e.g., into the femoral artery, thejugular vein, etc.) to monitor blood temperature and pressure and tothereby determine the cardiac output or pulmonary capillary blood flowof the patient. Although such techniques can produce a reasonablyaccurate result, the invasive nature of these procedures has a highpotential for causing morbidity or mortality.

[0006] Adolph Fick's formula for calculating cardiac output, which wasfirst proposed in 1870, has served as the standard by which other meansof determining cardiac output and pulmonary capillary blood flow havesince been evaluated. Fick's well-known equation, which is also referredto as the Fick Equation, written for carbon dioxide (CO₂), is:${Q = \frac{V_{{CO}_{2}}}{\left( {C_{v_{{CO}_{2}}} - C_{a_{{CO}_{2}}}} \right)}},$

[0007] where Q is cardiac output, VCO ₂ is the amount of CO₂ excreted bythe lungs, or “CO₂ elimination,” and Ca_(CO) ₂ and Cv_(CO) ₂ are the CO₂contents of arterial blood and venous blood, respectively. Notably, theFick Equation presumes an invasive method (i.e., catheterization) ofcalculating cardiac output or pulmonary capillary blood flow because thearterial blood and mixed venous blood must be sampled in order todirectly determine the CO₂ contents of arterial blood and venous blood.

[0008] It has been shown, however, that by using the principles embodiedin the Fick Equation, non-invasive means may be employed to determinecardiac output or pulmonary capillary blood flow. That is, expired CO₂levels, measured in terms of fraction of expired gases that comprise CO₂(f_(CO) ₂ ) or in terms of partial pressure of CO₂ (P_(CO) ₂ ), can bemonitored and employed to estimate the content of CO₂ in the arterialblood. Thus, a varied form of the Fick Equation may be employed toestimate cardiac output or pulmonary capillary blood flow based onobserved changes in f_(CO) ₂ or P_(CO) ₂ .

[0009] An exemplary use of the Fick Equation to non-invasively determinecardiac output or pulmonary capillary blood flow includes comparing a“standard” ventilation event to a change in expired CO₂ values and achange in excreted volume of CO₂, which is referred to as carbon dioxideelimination or CO₂ elimination (VCO ₂), which may be caused by a suddenchange in ventilation. Conventionally, a sudden change in effectiveventilation has been caused by having a patient inhale or breathe avolume of previously exhaled air. This technique is typically referredto as “re-breathing.”

[0010] Some re-breathing techniques have used the partial pressure ofend-tidal CO₂ (Pet_(CO) ₂ or et_(CO) ₂ ) to approximate the content ofCO₂ in the arterial blood of a patient while the patient's lungs act asa tonometer to facilitate the measurement of the CO₂ content of thevenous blood of the patient.

[0011] By further modification of the Fick Equation, it may be assumedthat the CO₂ content of the patient's venous blood does not changewithin the time period of the perturbation. Thus, the need to directlycalculate the CO₂ content of venous blood was eliminated by employingthe so-called “partial re-breathing” method. (See, Capek et al.,“Noninvasive Measurement of Cardiac Output Using Partial CO₂Rebreathing,” IEEE Transactions on Biomedical Engineering, Vol. 35, No.9, September 1988, pp. 653-661 (hereinafter “Capek”).)

[0012] The carbon dioxide elimination of the patient may benon-invasively measured as the difference per breath between the volumeof carbon dioxide inhaled during inspiration and the volume of carbondioxide exhaled during expiration, and is typically calculated as theintegral of the carbon dioxide signal times the rate of flow over anentire breath. The volume of carbon dioxide inhaled and exhaled may eachbe corrected for any deadspace or for any intrapulmonary shunt.

[0013] The partial pressure of end tidal carbon dioxide is also measuredin re-breathing processes. The partial pressure of end-tidal carbondioxide, after correcting for any deadspace, is typically assumed to beapproximately equal to the partial pressure of carbon dioxide in thealveoli (PA _(CO) ₂ ) of the patient or, if there is no intrapulmonaryshunt, the partial pressure of carbon dioxide in the arterial blood ofthe patient (Pa_(CO) ₂ ). Conventionally employed Fick methods ofdetermining cardiac output or pulmonary capillary blood flow typicallyinclude a direct, invasive determination of Cv_(CO) ₂ by analyzing asample of the patient's mixed venous blood. The re-breathing process istypically employed to either estimate the carbon dioxide content ofmixed venous blood (in total re-breathing) or to obviate the need toknow the carbon dioxide content of the mixed venous blood (by partialre-breathing) or determine the partial pressure of carbon dioxide in thepatient's venous blood (Pv_(CO) ₂ ).

[0014] Re-breathing processes typically include the inhalation of a gasmixture that includes carbon dioxide. During re-breathing, the carbondioxide elimination of a patient typically decreases. In totalre-breathing, carbon dioxide elimination decreases to near zero. Inpartial re-breathing, carbon dioxide elimination does not cease. Thus,in partial re-breathing, the decrease in carbon dioxide elimination isnot as large as that of total re-breathing.

[0015] Re-breathing can be conducted with a re-breathing circuit, whichcauses a patient to inhale a gas mixture that includes carbon dioxide.FIG. 1 schematically illustrates a conventional ventilation system thatis typically used with patients who require assisted breathing during anillness, during a surgical procedure, or during recovery from a surgicalprocedure. The conventional ventilator system 10 includes a tubularportion 12 that may be inserted into the trachea of a patient by knownintubation procedures. The end 14 (i.e., the end most distant from thepatient) of the tubular portion 12 may be fitted with a Y-piece 16 thatinterconnects an inspiratory hose 18 and an expiratory hose 20. Both theinspiratory hose 18 and expiratory hose 20 may be connected to aventilator machine (not shown), which delivers air into the breathingcircuit through the inspiratory hose 18. A one-way valve 22 ispositioned on the inspiratory hose 18 to prevent exhaled gas fromentering the inspiratory hose 18 beyond the valve 22. A similar one-wayvalve 24 on the expiratory hose 20 limits movement of inspiratory gasinto the expiratory hose 20. Exhaled air flows passively into theexpiratory hose 20.

[0016] With reference to FIG. 2, an exemplary known re-breathingventilation circuit 30 is shown. Re-breathing circuit 30 includes atubular portion 32 insertable into the trachea of a patient by knownintubation procedures. Gases may be provided to the patient from aventilator machine (not shown) via an inspiratory hose 34 interconnectedwith tubular portion 32 by a Y-piece 36. Tubular portion 32 and anexpiratory hose 38 are also interconnected by Y-piece 36. An additionallength of hose 40 is provided in flow communication with the tubularportion 32, between the tubular portion 32 and the Y-piece 36, and actsas a deadspace for receiving exhaled gas. A three-way valve 42,generally positioned between the Y-piece 36 and the opening to theadditional length of hose 40, is constructed for intermittent actuationto selectively direct the flow of gas into or from the additional lengthof hose 40. That is, at one setting, the valve 42 allows inspiratory gasto enter the tubular portion 32 while preventing movement of the gasinto the additional length of hose 40. At a second setting, the valve 42allows exhaled gas to enter into the expiratory hose 38 while preventingmovement of gas into the additional length of hose 40. At a thirdsetting, the three-way valve 42 directs exhaled air to enter into theadditional length of hose 40 and causes the patient to re-breathe theexhaled air on the following breath to, thereby, effect re-breathing andto cause a change in the effective ventilation of the patient.

[0017] The change in CO₂ elimination and in the partial pressure ofend-tidal CO₂ caused by the change in ventilation in the system of FIG.2 can then be used to calculate the cardiac output or pulmonarycapillary blood flow of the patient. Sensing and/or monitoring devicesmay be attached to the re-breathing ventilation circuit 30 between theadditional length of hose 40 and the tubular portion 32. The sensingand/or monitoring devices may include, for example, means 44 fordetecting CO₂ concentration and means 46 for detecting respiratory flowparameters during inhalation and exhalation. These sensing and/ormonitoring devices are typically associated with data recording anddisplay equipment (not shown). One problem encountered in use of theconventional re-breathing system is that the volume of the deadspaceprovided by the additional length of hose 40 is fixed and may not beadjusted. As a result, the amount of deadspace provided in the circuitfor a small adult to effect re-breathing is the same amount of deadspaceavailable for a large adult to effect re-breathing, and the resultingchanges in CO₂ values for patients of different sizes or breathingcapacities, derived from fixed-deadspace systems, can produce inadequateevaluation of a patient's cardiac output or pulmonary capillary bloodflow. Further, the three-way valve 42 of the system is expensive andsignificantly increases the cost of the ventilation device.

[0018] During total re-breathing, the partial pressure of end-tidalcarbon dioxide (Pet_(CO) ₂ ) is typically assumed to be equal to thepartial pressure of carbon dioxide in the venous blood (Pv_(CO) ₂ ) ofthe patient, as well as to the partial pressure of carbon dioxide in thearterial blood (Pa_(CO) ₂ ) of the patient and to the partial pressureof carbon dioxide in the alveolar blood (PA _(CO) ₂ ) of the patient.The partial pressure of carbon dioxide in blood may be converted to thecontent of carbon dioxide in blood by means of a carbon dioxidedissociation curve.

[0019] In partial re-breathing, measurements during normal breathing andsubsequent re-breathing are substituted into the carbon dioxide Fickequation. This results in a system of two equations and two unknowns(carbon dioxide content in the mixed venous blood and cardiac output),from which cardiac output or pulmonary capillary blood flow can bedetermined without knowing the carbon dioxide content of the mixedvenous blood (Cv_(CO) ₂ ).

[0020] Total re-breathing is a somewhat undesirable means of measuringcardiac output or pulmonary capillary blood flow because the patient isrequired to breathe directly into and from a closed volume of gases(e.g., a bag) in order to produce the necessary effect. Moreover, it istypically impossible or very difficult for sedated or unconsciouspatients to actively participate in inhaling and exhaling into a fixedvolume.

[0021] Known partial re-breathing methods are also advantageous overinvasive techniques of measuring cardiac output or pulmonary capillaryblood flow because partial re-breathing techniques are non-invasive, usethe accepted Fick principle of calculation, are easily automated, andfacilitate the calculation of cardiac output or pulmonary capillaryblood flow from commonly monitored clinical signals. However, knownpartial re-breathing methods are somewhat undesirable because they are aless accurate means of measuring the cardiac output or pulmonarycapillary blood flow of non-intubated or spontaneously breathingpatients, may only be conducted intermittently (usually at intervals ofat least about four minutes), and result in an observed slight, butgenerally clinically insignificant, increase in arterial CO₂ levels.Moreover, the apparatus typically employed in partial re-breathingtechniques do not compensate for differences in patient size orbreathing capacities. In addition, many devices employ expensiveelements, such as three-way valves, which render the devices tooexpensive to be used as disposable units.

[0022] Thus, there is a need for adjustable deadspace re-breathingapparatus that compensate for differences in the sizes or breathingcapacities of different patients, that may be employed to provide a moreaccurate and continuous measurement of gases exhaled or inhaled by apatient, and are less expensive than conventional re-breathing apparatusand, thereby, facilitate use of the adjustable deadspace re-breathingapparatus as a single-use, or disposable, product. There is also a needfor a more accurate method of estimating the cardiac output or pulmonarycapillary blood flow of a patient.

SUMMARY OF THE INVENTION

[0023] In accordance with the present invention, apparatus and methodsfor measuring the cardiac output or pulmonary capillary blood flow of apatient are provided. The apparatus of the present invention includes adeadspace (i.e., volume of re-breathed gases), the volume of which canbe adjusted without changing airway pressure. The invention alsoincludes methods of adjusting the volume of deadspace to obtain a moreaccurate cardiac output or pulmonary capillary blood flow value. Amodified form of the Fick Equation may be employed with the adjustabledeadspace volume to calculate the cardiac output or pulmonary capillaryblood flow of the patient. The apparatus of the present invention alsoemploys significantly less expensive elements of construction, therebyfacilitating the use of the apparatus as a disposable product.

[0024] The apparatus and methods of the present invention apply amodified Fick Equation to calculate changes in partial pressure ofcarbon dioxide (P_(CO) ₂ ), flow, and concentration to evaluate thecardiac output or pulmonary capillary blood flow of a patient. Thetraditional Fick Equation, written for CO₂ is:${Q = \frac{V_{{CO}_{2}}}{\left( {C_{v_{{CO}_{2}}} - C_{a_{{CO}_{2}}}} \right)}},$

[0025] where Q is pulmonary capillary blood flow (“PCBF”), V_(CO) ₂ isthe output of CO₂ from the lungs, or “CO₂ elimination,” and Ca_(CO) ₂and Cv_(CO) ₂ are the CO₂ contents of the arterial blood and venousblood CO₂, respectively. It has been shown in the prior work of othersthat cardiac output can be estimated from calculating the change in thefraction or volume of CO₂ exhaled by a patient and the partial pressureof end-tidal CO₂ as a result of a sudden change in ventilation. That canbe done by applying a differential form of the Fick Equation, asfollows:${Q = {\frac{V_{{CO}_{2_{1}}}}{\left( {C_{v1} - C_{a1}} \right)} = \frac{V_{{CO}_{2_{2}}}}{\left( {C_{v_{2}} - C_{a_{2}}} \right)}}},$

[0026] where Ca_(CO) ₂ is the CO₂ content of the arterial blood of apatient, Cv_(CO) ₂ is the CO₂ content of the venous blood of thepatient, and the subscripts 1 and 2 refer to measured values before achange in ventilation and measured values during a change inventilation, respectively. The differential form of the Fick Equationcan, therefore, be rewritten as:${Q = {{\frac{V_{{CO}_{2_{1}}} - V_{{CO}_{2_{2}}}}{\left( {C_{v1} - C_{a1}} \right) - \left( {C_{v2} - C_{a2}} \right)}\quad {or}\quad Q} = {\frac{\Delta \quad V_{{CO}_{2}}}{\Delta \quad C_{a_{{CO}_{2}}}} = \frac{\Delta \quad V_{{CO}_{2}}}{s\quad \Delta \quad {{Pet}{CO}}_{2}}}}},$

[0027] where ΔVCO ₂ is the change in CO₂ elimination in response to thechange in ventilation, ΔCa_(CO) ₂ is the change in the CO₂ content ofthe arterial blood of the patient in response to the change inventilation, ΔPet_(CO) ₂ is the change in the partial pressure ofend-tidal CO₂, and s is the slope of a CO₂ dissociation curve known inthe art. The foregoing differential equation assumes that there is noappreciable change in venous CO₂ concentration during the re-breathingepisode, as demonstrated by Capek. Also, a CO₂ dissociation curve, wellknown in the art, is used for determining CO₂ concentration based onpartial pressure measurements.

[0028] In previous partial re-breathing methods, a deadspace, which maycomprise an additional 50-250 ml capacity of air passage, was providedin the ventilation circuit to decrease the effective alveolarventilation. In the present invention, a ventilation apparatus isprovided with a deadspace having an adjustable volume to provide achange in ventilation for determining accurate changes in CO₂elimination and in partial pressure of end-tidal CO₂ that iscommensurate with the requirements of patients of different sizes orbreathing capacities. In one embodiment of the ventilation apparatus,selectively adjustable deadspace is provided into which the patient mayexhale and from which the patient may inhale. Thus, the adjustabledeadspace volume of the apparatus accommodates a variety of patientsizes or breathing capacities (e.g., from a small adult to a largeadult). As a result, the patient is provided with a volume ofre-breathable gas commensurate with the patient's size or breathingcapacity, which decreases the effective ventilation of the patientwithout changing the airway pressure of the patient. Because airway andintra-thoracic pressure are not affected by the re-breathing method ofthe present invention, cardiac output and pulmonary capillary blood floware not significantly affected by re-breathing.

[0029] In an alternative method, the volume of deadspace may beeffectively lessened by selectively leaking exhaled gas from theventilation system to atmosphere or to a closed receptacle means duringinspiration. Similarly, additional carbon dioxide may be introduced intothe deadspace to increase the effective deadspace volume. Changing theeffective deadspace volume in such a manner has substantially the sameeffect as changing the actual volume of the deadspace of the ventilationapparatus.

[0030] The ventilation apparatus of the present invention includes atubular portion, which is also referred to as a conduit, to be placed inflow communication with the airway of a patient. The conduit of theventilation apparatus may also be placed in flow communication with orinclude an inhalation course and an exhalation course, each of which mayinclude tubular members or conduits. In a common configuration, theinhalation course and exhalation course may be interconnected in flowcommunication between a ventilator unit (i.e., a source of deliverablegas mechanically operated to assist the patient in breathing) and thepatient. Alternatively, however, a ventilator unit need not be used withthe ventilation apparatus. For example, inhaled air and exhaled air maybe taken from or vented to atmosphere. Other conventional equipmentcommonly used with ventilator units or used in ventilation of a patient,such as a breathing mask, may be used with the inventive ventilationapparatus.

[0031] A pneumotachometer for measuring gas flow and a capnometer formeasuring CO₂ partial pressure are provided along the flow path of theventilation apparatus and, preferably, in proximity to the conduit,between the inhalation and exhalation portions of the ventilationapparatus and the patient's lungs. The pneumotachometer and capnometerdetect changes in gas concentrations and flow and are preferably inelectrical communication with a computer programmed (i.e., by softwareor embedded hardware) to store and evaluate, in substantially real time,the measurements taken by the detection apparatus. Other forms ofdetection apparatus may, alternatively or in combination with thepneumotachometer and the capnometer, be employed with the ventilationapparatus of the present invention.

[0032] Deadspace having an adjustable volume is provided in flowcommunication with the conduit. In particular, the deadspace is in flowcommunication with the exhalation portion of the ventilation apparatus(e.g., the expiratory course), and may be in flow communication with theinhalation portion (e.g., the inspiratory course) of the ventilationapparatus. In one embodiment, the volume of the deadspace may bemanually adjusted. Alternatively, electromechanical means may beoperatively associated with the computer and with the deadspace toprovide automatic adjustment of the volume of the deadspace in responseto the patient's size or breathing capacity or in response to changes inthe ventilation or respiration of the patient.

[0033] In an alternative embodiment, a tracheal gas insufflation (“TGI”)apparatus is employed to provide the change in ventilation necessary todetermine pulmonary CO₂ changes and to determine the cardiac output orpulmonary capillary blood flow of a patient in accordance with thedifferential Fick Equation disclosed previously. Tracheal gasinsufflation apparatus are known, and are typically used to flush thedeadspace of the alveoli of the lungs and to replace the deadspace withfresh gas infused through the TGI apparatus. That is, fresh gas isintroduced to the central airway of a patient to improve alveolarventilation and/or to minimize ventilatory pressure requirements. A TGIapparatus may be interconnected, for example, by means of a catheter,with a ventilator apparatus and includes a means of introducing freshgas into the breathing tube and into the lungs of the patient. The TGIapparatus may be used in the methods of the present invention todetermine baseline measurements of CO₂ elimination, partial pressure ofend tidal CO₂, or partial pressure of alveolar CO₂ during TGI. When theTGI system is turned off, a deadspace is formed by the patient's tracheaand the endo-tracheal tube of the TGI apparatus, which facilitatesmeasurement of a change in the partial pressure of CO₂ and in the amountof CO₂ eliminated by the patient that may be evaluated in accordancewith the method of the present invention. Further, the catheter of theTGI apparatus may be variably positioned within the trachea of thepatient to further adjust the deadspace volume.

[0034] During re-breathing, the deadspace provided by the apparatus ofthe present invention facilitates a rapid drop in CO₂ elimination, whichthereafter increases slightly and slowly as the functional residual lunggas capacity, which is also referred to as functional residual capacityor “FRC,” equilibrates with the increase in the partial pressure of CO₂in the alveoli. Partial pressure of end tidal CO₂ increases at a slowerrate than CO₂ elimination following the addition of deadspace, dependingon alveolar deadspace and the cardiac output or pulmonary capillaryblood flow of the patient, but then stabilizes to a new level. A“standard,” or baseline, breathing episode is conducted for a selectedperiod of time immediately preceding the introduction of a deadspaceinto the breathing circuit (i.e., immediately preceding re-breathing)and CO₂ elimination and partial pressure of end tidal CO₂ values aredetermined based on measurements made during the “standard” breathingevent. These values are substituted as the values VCO ₂ and Ca_(CO) ₂ inthe differential Fick Equation. Carbon dioxide elimination and partialpressure of end tidal CO₂ values are also determined from measurementstaken for a predetermined amount of time (e.g., approximately thirtyseconds) following the introduction of a deadspace (i.e., after theonset of re-breathing) during partial re-breathing to provide the secondset of values (subscript 2 values) in the differential Fick Equation.Thus, the predetermined amount of time at which the second set of valuesare obtained may be about the same as the duration of partialre-breathing. The period of time during which partial re-breathingoccurs and during which normal breathing occurs may be determined by theindividual patient's size and breathing capacity. Additionally, theperiod of time between a re-breathing episode and a subsequent normalbreathing episode may vary between patients, depending on a particularpatient's size and breathing capacity.

[0035] Cardiac output or pulmonary capillary blood flow may bedetermined in accordance with the method of the present invention byestimating the partial pressure of CO₂ in the alveoli or the content ofthe blood in capillaries that surround the alveoli of the lungs of apatient (Cc′_(CO) ₂ ), or the alveolar CO₂ content (CA _(CO) ₂ ), ratherthan basing the cardiac output or pulmonary capillary blood flowdetermination on the partial pressure of end-tidal CO₂, as is typicallypracticed in the art. Partial pressure values that are obtained from CO₂measurements are converted to a value for gas content in the blood usinga CO₂ dissociation curve or equation, as known in the art. Thus, a moreaccurate cardiac output or pulmonary capillary blood flow value can bedetermined with alveolar CO₂ measurements than with partial pressure ofend tidal CO₂ measurements.

[0036] In addition, the accuracy of the cardiac output or pulmonarycapillary blood flow measurement may be increased by correcting CO₂elimination values to account for flow of CO₂ into the functionalresidual capacity of the lungs, which is the volume of gas that remainsin the lungs at the end of expiration. The cardiac output or pulmonarycapillary blood flow of the patient may then be determined by accountingfor the functional residual capacity and by employing the valuesobtained in accordance with the method of the present invention, as wellas other determined values, known values, estimated values, or any othervalues based on experiential data, such as by a computer processor inaccordance with the programming thereof. Alternatively, cardiac outputor pulmonary capillary blood flow may be estimated without accountingfor functional residual capacity.

[0037] The ventilation apparatus of the present invention may alsoemploy inexpensive yet accurate monitoring systems as compared to thesystems currently used in the art. The methods of the invention mayinclude the automatic adjustment of the deadspace volume of theapparatus to accommodate patients of different sizes or breathingcapacities or changes in the ventilation or respiration of a patient,and provides consistent monitoring with modest recovery time. Further,the present apparatus and methods can be used with non-responsive,intubated patients and with non-intubated, responsive patients.

[0038] Other features and advantages of the present invention willbecome apparent through a consideration of the ensuing description, theaccompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0039]FIG. 1 is a schematic representation of a conventional ventilationsystem used to assist patient breathing;

[0040]FIG. 2 is a schematic representation of a conventionalre-breathing system;

[0041]FIG. 3 is a schematic representation of a first embodiment of theventilation apparatus of the present invention, illustrating a deadspacewith an adjustably expandable volume;

[0042]FIG. 4 is a schematic representation of an alternative embodimentof the present invention, wherein the re-breathing circuit isconstructed with an evacuation valve;

[0043] FIGS. 5A-5C are schematic representations of another alternativeembodiment of the present invention, wherein the inspiratory course andexpiratory course of the breathing circuit are interconnected and thebreathing circuit includes a two-way valve closeable across a flow pathof the breathing circuit;

[0044]FIG. 6 is a schematic representation of an alternative embodimentsimilar to the embodiment shown in FIGS. 5A-5C, wherein the volumes ofthe inspiratory course and expiratory course of the breathing circuitare adjustably expandable;

[0045]FIG. 7 is a schematic representation of another embodiment of theinvention, wherein a series of valves is provided along the length ofthe inspiratory course and expiratory course of the breathing circuit toprovide a selectable volume of deadspace dependent upon the size,breathing capacity, or changes in the ventilation or respiration of thepatient;

[0046]FIGS. 8A and 8B are schematic representations of anotherembodiment of the invention, wherein an evacuation valve is providedwith a vent to atmosphere and to a receptacle or chamber, respectively;

[0047]FIG. 9 is a schematic representation of a breathing circuit of thepresent invention that includes a tracheal gas insufflation apparatus,which can be used to provide a volume of deadspace;

[0048]FIG. 10 is a schematic representation of human lungs, illustratingthe concepts of parallel deadspace, alveolar deadspace and serialdeadspace in the lungs of a patient;

[0049]FIG. 11 is a flow diagram that illustrates the calculations madein the method of the present invention to determine cardiac output orpulmonary capillary blood flow by employing the measured values duringboth normal breathing and partial re-breathing; and

[0050]FIG. 12 is a schematic representation of a variation of theventilation apparatus of FIG. 7, including sections having selectivelyexpandable volumes.

DETAILED DESCRIPTION OF THE INVENTION

[0051] Ventilation Apparatus and Methods

[0052]FIG. 3 illustrates a breathing circuit of the present invention,which is illustrated as a ventilation apparatus 50, and which may beemployed to determine the cardiac output of pulmonary capillary bloodflow of a patient. Ventilation apparatus 50 comprises a tubular airway52, which is also referred to as an airway conduit or simply as aconduit, that may be placed in flow communication with the trachea orlungs of the patient. The present ventilation apparatus 50 may be placedin flow communication with the trachea of the patient by knownintubation procedures or by positioning a breathing mask over the noseand/or mouth of the patient. Ventilation apparatus 50 may be used withunconscious or uncooperative patients needing ventilation assistance,and may be used with substantially equal efficacy with patients who areconscious. Ventilation apparatus 50 may also include an inspiratory hose54, which is also referred to as an inspiratory course or as aninspiration portion of the breathing circuit, and an expiratory hose 56,which is also referred to as an expiratory course or as an expirationportion of the breathing circuit, both of which are in substantial flowcommunication with tubular airway 52. The inspiratory hose 54 and theexpiratory hose 56 may each be ventilated to atmosphere or operativelyconnected to a ventilator machine 55 to facilitate the delivery of air,breathing gases, or other breathing medium to the patient through theinspiratory hose 54. The inspiratory hose 54 and expiratory hose 56 mayeach be joined in flow communication with the tubular airway 52 by meansof a Y-piece 58.

[0053] An additional length of conduit or hose 60, which provides adeadspace volume for receiving exhaled gas from the patient, ispreferably in flow communication with the tubular airway 52. Both endsof the additional length of hose 60 are preferably in flow communicationwith tubular airway 52. The additional length of hose 60 is configuredto be selectively expandable to readily enable the volume of deadspaceto be adjusted commensurate with the size or breathing capacity of thepatient, or commensurate with changes in the ventilation or respirationof the patient, such as an increased or decreased tidal volume ormodified respiration rate. As suggested by FIG. 3, selective expansionof the deadspace may be accomplished by configuring the additionallength of hose 60 to include an expandable section 62 made of, forexample, a section of corrugated hose which can be lengthened orshortened by simply pulling or pushing the expandable section 62substantially along its longitudinal axis 64. The section of corrugatedhose will preferably retain the length to which it is set until adjustedagain. Other suitable means of providing adjustable expansion of thevolume of the deadspace and, thus, methods of adjusting the volume ofthe deadspace of the breathing circuit are also available and within thescope of the present invention.

[0054] A three-way valve 68 may be disposed along the flow path oftubular airway 52 between the two ends of additional length of hose 60and selectively positioned to direct inspiratory gas into a deadspace 70comprised of the additional length of hose 60 upon inhalation, toselectively prevent exhaled gas from entering the deadspace 70 duringnormal breathing, or to direct exhaled gas into deadspace 70 duringre-breathing so that the patient will re-breathe previously exhaledgases or a gas including CO₂ from the deadspace 70.

[0055] A flow meter 72, such as a pneumotachometer, and a carbon dioxidesensor 74, which is typically referred to as a capnometer, may beexposed to the flow path of the ventilation apparatus, preferablybetween the tubular airway 52 and the additional length of hose 60.Thus, the flow meter 72 and carbon dioxide sensor 74 are exposed to anyair or gas that flows through ventilation apparatus 50. The flow meter72 detects gas flow through the ventilation apparatus 50.

[0056] A flow meter 72 of a known type, such as thedifferential-pressure type respiratory flow sensors manufactured byNovametrix Medical Systems Inc. (“Novametrix”) of Wallingford, Conn.(e.g., the Pediatric/Adult Flow Sensor (Catalog No. 6717) or theNeonatal Flow Sensor (Catalog No. 6718)), which may be operativelyattached to a ventilation apparatus (not shown), as well as respiratoryflow sensors based on other operating principles and manufactured ormarketed by others, may be employed to measure the flow rates of thebreathing of the patient.

[0057] The carbon dioxide sensor 74 detects CO₂ levels and, therefore,facilitates a determination of changes in CO₂ levels that result fromchanges in the ventilation or respiration of the patient. The carbondioxide sensor 74 and its associated airway adapter may be an “onairway” sensor, a sampling sensor of the type which withdraws a sidestream sample of gas for testing, or any other suitable type of carbondioxide sensor. Exemplary carbon dioxide sensors and complementaryairway adapter include, without limitation, the Pediatric/Adult SinglePatient Use Airway Adapter (Catalog No. 6063), the Pediatric/AdultReusable Airway Adapter (Catalog No. 7007), or the Neonatal/PediatricReusable Airway Adapter (Catalog No. 7053), which are manufactured byNovametrix. Alternatively, combined flow and carbon dioxide sensors, asknown in the art, may be employed.

[0058] The data obtained by the flow meter 72 and by the carbon dioxidesensor 74 are preferably used to determine the cardiac output orpulmonary capillary blood flow of the patient. Accordingly, the flowmeter 72 and carbon dioxide sensor 74 may be operatively associated witha computer 76 (e.g., by direct cable connection, wireless connection,etc.) programmed to store or analyze data from the flow meter 72 and thecarbon dioxide sensor 74 and programmed to determine the cardiac outputor pulmonary capillary blood flow of the patient from the stored oranalyzed data.

[0059] As previously described herein, the differential Fick Equationrequires a change in the partial pressure of carbon dioxide and a changein carbon dioxide elimination to be induced in the patient in order toestimate the cardiac output or pulmonary capillary blood flow of thepatient. As the patient re-breathes previously exhaled gas, the amountof CO₂ inhaled by the patient increases, thereby facilitating theevaluation of increased CO₂ levels during a change in effectiveventilation, as compared to the CO₂ levels of the patient's breathingduring normal ventilation. The re-breathing ventilation apparatus 50 ofthe present invention provides the ability to selectively adjust thevolume of deadspace from which air is re-breathed in accordance with thesize or breathing capacity of the patient, or in response to changes inthe ventilation or respiration of the patient. For example, if thedetected change in partial pressure of end tidal CO₂ is less than athreshold pressure (e.g., 1 mm Hg), or the change in CO₂ elimination isless than a threshold percentage or fraction (e.g., 20% or 0.2) of abaseline CO₂ elimination, then the deadspace volume may be increased byan appropriate amount (e.g., 20%). Similarly, if the detected change inpartial pressure of end tidal CO₂ is greater than a threshold pressure(e.g., 12 mm Hg), or the change in CO₂ elimination is greater than athreshold percentage or fraction (e.g., 80% or 0.8) of the baseline CO₂elimination, then the deadspace volume may be decreased by anappropriate amount (e.g., 20%).

[0060] In an alternative embodiment of the re-breathing ventilationapparatus 50′ of the invention, as shown in FIG. 4, the expense of usinga three-way valve may be eliminated by disposing an inexpensive two-wayvalve 78′ along the flow path of the additional length of hose 60′ andby positioning a flow restrictor 80 (e.g., a region of tubular airway 52of decreased inner diameter) along tubular airway 52 between the inlet82 and outlet 84 (i.e., the two ends) of the additional length of hose60′. Thus, when the two-way valve 78′ is closed, inhaled and exhaledgases will be directed through the flow restrictor 80. Duringre-breathing, the two-way valve 78′ is placed in an open position sothat the exhaled air, encountering the flow restrictor 80, follows thecourse of less resistance into the deadspace 70′. Inhaled, re-breathedair similarly follows the course of least resistance and flows from thedeadspace 70′. As the optimal amount of air re-breathed by the patientmay depend upon the size, breathing capacity, or changes in theventilation or respiration of the patient, or on another factor, it maybe desirable to adjust the deadspace 70′ at the expandable section 62′to provide the necessary volume of deadspace for determining the cardiacoutput or pulmonary capillary blood flow of the patient.

[0061] In another alternative embodiment of the ventilation apparatus50″ of the present invention, as shown in FIGS. 5A-5C, a shunt line 85is positioned between the inspiratory course 54″ and the expiratorycourse 56″ to provide a selectively-sized deadspace 70″ in there-breathing circuit. In the configuration of the embodiment shown inFIGS. 5A-5C, the inspiratory course 54″ and expiratory course 56″ maycomprise at least a part of the deadspace 70″. A two-way shunt valve 86,positioned in the flow path of the shunt line 85, selectively directsthe flow of inspired and expired gas, dependent upon whether the shuntvalve 86 is placed in an open position or a closed position. Thus, whenthe ventilation apparatus 50″ is configured for normal or baselinebreathing, as depicted in FIG. 5A, exhaled air (represented by theshaded area) will enter the expiratory course 56″. During normalbreathing, the shunt valve 86 is placed in the closed position. During are-breathing episode, as depicted in FIG. 5B, the shunt valve 86 isplaced in the open position, and exhaled gas may fill a portion of theinspiratory course 54″, substantially all of the expiratory course 56″,and the shunt line 85, all of which serve as the deadspace 70″.

[0062] The deadspace 70″ in the embodiment shown in FIGS. 5A-5C may berendered further expandable, as shown in FIG. 6, by structuring theinspiratory course 54″ with an expandable section 90 positioned betweenthe shunt line 85 and the Y-piece 58, and/or by structuring theexpiratory course 56″ with an expandable section 92 positioned betweenthe shunt line 85 and the Y-piece 58. Thus, the deadspace 70″ can beselectively adjusted in accordance with the size or capacity of thepatient, or responsive to operating conditions, by increasing ordecreasing the volume of the expandable sections 90, 92 of theinspiratory course 54″ and expiratory course 56″, respectively. Shuntline 85 may similarly include a volume expandable section. As explainedpreviously in reference to FIG. 3, any suitable adjustably expandablemeans may be employed as expandable sections 90, 92. For example, asdepicted in FIG. 6, the expandable sections 90, 92 may be fabricatedfrom a corrugated plastic material, the length of which can be easilyexpanded or contracted and preferably substantially maintained untilre-adjusted. The embodiment of FIG. 6 provides a particularly simple andinexpensive construction that may render it easy-to-use and facilitateits use as a disposable product.

[0063] In yet another embodiment of the ventilation apparatus 50′″ ofthe present invention, as shown in FIG. 7, a plurality of shunt lines85′″, 94, 96 is positioned between the inspiratory course 54′″ and theexpiratory course 56′″, with each shunt line 85′″, 94, 96 including atwo-way shunt valve 86′″, 98, 100, respectively, disposed along the flowpath thereof. In operation, the amount of deadspace 70′″ desired,according to the size or breathing capacity of the patient or otherfactors, may be selectively adjusted by permitting exhaled gas to movethrough any suitable combination of shunt lines 85′″, 94, 96. Forexample, given a patient of average size or lung capacity, it may beappropriate to use shunt line 85′″ and shunt line 94 as potentialdeadspace 70′″. Thus, as the patient exhales in a re-breathing episode,the shunt valves 86′″, 98 associated with shunt line 85′″ and shunt line94, respectively, may be placed in an open position to permit exhaledand re-breathable gas to fill the expiratory course 56′″, theinspiratory course 54′″ between shunt line 94 and the Y-piece 58, shuntline 85′″, and shunt line 94. With a patient of larger size or greaterlung capacity, it may be necessary to use the third shunt line 96 toprovide sufficient deadspace 70′″ for re-breathing. Notably, each shuntvalve 86′″, 98, 100 may be in electromechanical communication with thecomputer 76 (see FIG. 3) so that the computer may determine, from thecarbon dioxide sensor 74 (see FIG. 3), for example, that a differentvolume of deadspace 70′″ is needed. The computer 76 may then direct theopening or closing of one or more of the shunt valves 86′″, 98, 100 toprovide a sufficient volume of deadspace 70′″.

[0064] Referring to FIG. 12, as a variation of the embodimentillustrated in FIG. 7, the ventilation apparatus 50′″ may also includeselectively expandable sections 90′″, 92′″ similar to those shown inFIG. 6. Although expandable sections 90′″ and 92′″ are illustrated asbeing disposed along inspiratory course 54′″ and expiratory course 56′″,sections of expandable volume may also be disposed along other portionsof the potential deadspace of the breathing circuit, such as along anyof shunt lines 85′″, 94, or 96.

[0065] In the several embodiments of the invention previouslyillustrated and described, the amount or volume of the deadspace hasbeen selectively adjustable by providing means for adjusting the volumeof the deadspace, such as by providing length-expanding means. It may beequally appropriate, however, to provide a change in ventilation, asrequired by the differential Fick Equation, by leaking some of theexhaled gas out of the system during the inspiration phase of a breathor by increasing the level of CO₂ in the deadspace, both of whichprovide an effective change in the volume of deadspace. Thus, asillustrated by FIG. 8A, the ventilation apparatus 50 of the presentinvention may include an evacuation element or component. The evacuationelement may include an evacuation line 106 in flow communication with atleast the expiratory course 56 of the ventilation apparatus 50. Theevacuation element includes a structure that permits gas or anotherbreathing medium to flow into or out of the ventilation apparatus 50,such as an evacuation valve 108, which is also referred to as a valve,that, when opened, allows exhaled gas to escape the ventilationapparatus 50 through an orifice 110 positioned at the end of theevacuation line 106 or permits gas to be introduced into the ventilationapparatus 50. Alternatively, a valve may be positioned in flowcommunication with ventilation apparatus 50 to facilitate the flow ofgases therefrom.

[0066] The volume of exhaled gas that should be leaked from theventilation apparatus 50 or introduced therein during a re-breathingevent, as well as the timing and duration of such leakage orintroduction, may be determined by the computer 76 (see FIG. 3) inresponse to flow conditions, CO₂ conditions, the size or breathingcapacity of the patient, or changes in the ventilation or breathing ofthe patient. In addition, the evacuation valve 108, which may be inelectromechanical communication with the computer 76, may be selectivelyactuated by the computer 76 in accordance with the flow conditions, theCO₂ conditions, the size or breathing capacity of the patient, orchanges in the ventilation or respiration of the patient.

[0067] With reference to FIG. 8B, where a patient is anesthetized or isotherwise exhaling gas which is undesirable for venting to theatmosphere, a chamber or receptacle 112, such as an expandable bag, maybe disposed along the evacuation line 106 or otherwise in flowcommunication with the evacuation valve 108 to receive the exhaled gasleaked from the ventilation circuit.

[0068]FIG. 9 schematically illustrates the use of a tracheal gasinsufflation (TGI) apparatus 120 to provide the necessary deadspace indetermining the cardiac output or pulmonary capillary blood flow of apatient. TGI apparatus 120 are typically used to ventilate sick patientswho require the injection of fresh gas into their central airway toimprove alveolar ventilation. TGI apparatus can be configured to providecontinuous or phasic (e.g., only during inhalation) injections of gas.The TGI apparatus supplies gas, or an oxygen/gas mixture, to the lungswith every breath. As shown in FIG. 9, the TGI apparatus comprises anendotracheal tube 122, which may be inserted into the trachea 124 of thepatient by known intubation procedures. A catheter 126 extends throughthe endotracheal tube 122 and into the patient's lungs, typically justabove the carina. Gas or an oxygen/gas blend is provided from a gassource 128 and is directed through gas delivery tubing 130 into thecatheter 126. A flow meter 132 disposed along gas delivery tubing 130and in flow communication therewith may assist in determining theoptimum amount of gas to be introduced into the lungs.

[0069] An adaptor fitting 134 may be used to connect a ventilationapparatus 136, such as the type previously described in reference toFIGS. 1-8(B), to the TGI apparatus 120. That is, the ventilationapparatus 136 may include a Y-piece 58 from which an inspiratory course54 and an expiratory course 56 extend. The ventilation apparatus 136 mayalso include a flow meter 72 and a carbon dioxide sensor 74 disposed inflow communication therewith to collect data during normal breathing andduring a re-breathing event. In the illustrated TGI apparatus 120, theendotracheal tube 122 provides a volume of deadspace that may berequired for re-breathing in addition to any deadspace volume providedby the ventilation apparatus 136. In order to act as a deadspace,however, the TGI apparatus (i.e., the gas source 128 and flow meter 132)is preferably turned off, the amount of insufflation reduced, or the TGIapparatus otherwise disabled. Exhaled air is thereby allowed to flowinto the endotracheal tube 122 and, preferably, through the Y-piece 58.The endotracheal tube 122 and ventilation apparatus 136 or portionsthereof may then serve as deadspace. The volume of deadspace provided bythe TGI apparatus 120 may be further increased or decreased, asnecessary, by varying the depth to which the catheter 126 is positionedin the patient's trachea.

[0070] A computer 76 (see FIG. 3) to which the flow meter 72 and thecarbon dioxide sensor 74 may be connected can be programmed to receivedata from the flow meter 72 and the carbon dioxide sensor 74 and toanalyze the data to determine or estimate the cardiac output orpulmonary capillary blood flow of the patient.

[0071] Methods of Determining Cardiac Output or Pulmonary CapillaryBlood Flow

[0072] The determination of cardiac output or pulmonary capillary bloodflow for a given patient may be based on data obtained with the flowmonitor and the carbon dioxide sensor that are associated with theventilation apparatus of the present invention. Raw flow and CO₂ signalsfrom the flow monitor and the carbon dioxide sensor may be filtered toremove any artifacts, and the flow signals and CO₂ signals (e.g., dataregarding partial pressure of CO₂) may be stored by the computer 76.

[0073] Each breath, or breathing cycle, of the patient may bedelineated, as known in the art, such as by continually monitoring theflow rate of the breathing of the patient.

[0074] For each breathing cycle, the partial pressure of end-tidal CO₂carbon dioxide elimination (VCO ₂), the fraction of inspired, or “mixedinspired,” CO₂ and the airway deadspace are calculated. End-tidal CO₂ ismeasured, as known in the art. Carbon dioxide elimination is typicallycalculated as the integral of the respiratory flow over a breathingcycle (in milliliters) multiplied by the fraction of CO₂ over the entirebreath. The fraction of inspired CO₂ is the integral of CO₂ fractiontimes the air flow during inspiration, divided by the volume (inmilliliters) of inspired gas.

[0075] The values of VCO ₂ and Pet_(CO) ₂ may be filtered by employing amedian filter, which uses a median value from the most recent value ofrecorded VCO ₂ and Pet_(CO) ₂ values and the two values that precede themost recent measured value, as known in the art.

[0076] Preferably, when calculating VCO ₂, the VCO ₂ value is correctedto account for anatomic deadspace and alveolar deadspace. With referenceto FIG. 10, the lungs 150 of a patient may be described as including atrachea 152, two bronchi 154 and numerous alveoli 160, 162. Theanatomic, or “serial,” deadspace of lungs 150 includes the volume of thetrachea 152, bronchi 154, and other components of lungs 150 which holdgases, but do not participate in gas exchange. The anatomic deadspaceexists approximately in the region located between arrows A and B. Theso-called “shunted” blood bypasses pulmonary capillaries by way of anintrapulmonary shunt 165.

[0077] Lungs 150 typically include alveoli 160 that are in contact withblood flow and which can facilitate oxygenation of the blood, which arereferred to as “perfused” alveoli, as well as unperfused alveoli 162.Both perfused alveoli 160 and unperfused alveoli 162 may be ventilated.The volume of unperfused alveoli is the alveolar deadspace.

[0078] Perfused alveoli 160 are surrounded by and in contact withpulmonary capillaries 164. As deoxygenated blood 166 enters pulmonarycapillaries 164, oxygen binds to the hemoglobin molecules of the redblood cells of the blood, and CO₂ is released from the hemoglobin. Bloodthat exits pulmonary capillaries 164 in the direction of arrow 171 isreferred to as oxygenated blood 168. In alveoli 160 and 162, a volume ofgas known as the functional residual capacity (FRC) 170 remainsfollowing exhalation. The alveolar CO₂ is expired from a portion 172 ofeach of the alveoli 160 that is evacuated, or ventilated, duringexhalation.

[0079] The ventilated portion 178 of each of the unperfused alveoli 162may also include CO₂. The CO₂ of ventilated portion 178 of each of theunperfused alveoli 162, however, is not the result of O₂ and CO₂exchange in that alveolus. Since the ventilated portion 178 of each ofthe unperfused alveoli 162 is ventilated in parallel with the perfusedalveoli, ventilated portion 178 is typically referred to as “parallel”deadspace (PDS). Unperfused alveoli 162 also include a FRC 176, whichincludes a volume of gas that is not evacuated during a breath.

[0080] In calculating the partial pressure of CO₂ in the alveoli (PA_(CO) ₂ ) of the patient, the FRC and the partial pressure of CO₂ in theparallel deadspace in each of the unperfused alveoli 162 is preferablyaccounted for. FRC may be estimated as a function of body weight and ofthe airway deadspace volume by the following equation:

FRC=FRC−factor·(airway deadspace+offset value),

[0081] where FRC-factor is either an experimentally determined value oris based on published data (e.g., “experiential” data) known in the art,and the offset value is a fixed constant which compensates for breathingmasks or other equipment components that may add deadspace to thebreathing circuit and, thereby, unacceptably skew the relationshipbetween FRC and deadspace.

[0082] The partial pressure of CO₂ in the parallel dead space(CO_(2 PDS)) may be calculated from the mixed inspired CO₂ (Vi_(CO) ₂ )added to the product of the serial deadspace multiplied by the end tidalCO₂ of the previous breath (Pet_(CO) ₂ (n−1)). Because the averagepartial pressure of CO₂ in the parallel deadspace is equal to thepartial pressure of CO₂ in the parallel deadspace divided by the tidalvolume (V_(t)) (i.e., the total volume of one respiratory cycle, orbreath), the partial pressure of CO₂ in the parallel deadspace may becalculated on a breath-by-breath basis, as follows: $\begin{matrix}{{P_{{CO}_{2}\quad {PDS}}(n)} = {{\left\lbrack {{FRC}/\left( {{FRC} + V_{t}} \right)} \right\rbrack \cdot {P_{{CO}_{2}\quad {PDS}}\left( {n - 1} \right)}} +}} \\{\left( {P_{bar} \cdot \left( {\left( \left\lbrack {{Vi}_{{CO}_{2}} + {{deadspace} \cdot {{{Pet}_{{CO}_{2}}\left( {n - 1} \right)}/P_{bar}}}} \right) \right\rbrack/} \right.} \right.} \\{\left. \left. {\left. V_{t} \right) \cdot \left\lbrack {V_{t}/\left( {V_{t} + {FRC}} \right)} \right\rbrack} \right) \right),}\end{matrix}$

[0083] where (n) indicates a respiratory profile parameter (in thiscase, the partial pressure of CO₂ in the parallel deadspace, P_(CO) ₂_(PDS)(n)) from the most recent breath and (n−1) indicates a respiratoryprofile parameter from the previous breath.

[0084] The partial pressure of end-tidal CO₂, which is assumed to besubstantially equal to a weighted average of the partial pressure of CO₂in all of the perfused and unperfused alveoli of a patient, may becalculated as follows:

Pet _(CO) ₂ =®·P A _(CO) ₂ )+(1−r)P _(CO) _(2 PDS) ,

[0085] where r is the perfusion ratio, which is calculated as the ratioof perfused alveolar ventilation to the total alveolar ventilation, or(V_(A)−V_(PDS))/V_(A). The perfusion ratio may be assumed to be about0.95 or estimated, as known in the art. Alternatively, the perfusionratio may be determined by comparing arterial P_(CO) ₂ , whichmeasurement may be obtained directly from arterial blood and assumed tobe substantially the same as alveolar P_(CO) ₂ , to end tidal P_(CO) ₂values by rearranging the previous equation as follows:

r=(Pet _(CO) ₂ −P _(CO) _(2 PDS) )/(P A _(CO) ₂ −P _(CO) _(2 PDS) ).

[0086] By rearranging the preceding Pet_(CO) ₂ equation, the alveolarCO₂ partial pressure of the patient may be calculated. Preferably,alveolar CO₂ partial pressure is calculated from the end-tidal CO₂ andthe CO₂ in the parallel deadspace, as follows:

P A _(CO) ₂ =[Pet _(CO) ₂ −(1−r)P _(CO) ₂ _(PDS) ]/r.

[0087] The alveolar CO₂ partial pressure may then be converted toalveolar blood CO₂ content (CA _(CO) ₂ ) using an equation, such as thefollowing:

C A _(CO) ₂ =(6.957·Hb_(cone)+94.864)·ln(1+0.1933(P A _(CO) ₂ )),

[0088] where CA _(CO) ₂ is the content of CO₂ in the alveolar blood andHb is the concentration of hemoglobin in the blood of the pulmonarycapillaries. J. M. Capek and R. J. Roy, IEEE Transactions on BiomedicalEngineering (1988) 35(9):653-661. In some instances, a hemoglobin countand, therefore, the hemoglobin concentration, are available and may beemployed in calculating the CO₂ content. If a hemoglobin count orconcentration is not available, another value that is based uponexperiential or otherwise known data (e.g., 11.0 g/dl) may be employedin calculating the alveolar CO₂ content.

[0089] In calculating VCO ₂, the FRC and alveolar deadspace of the lungsof a patient may be accounted for by multiplying the FRC by the changein end tidal partial pressure, such as by the following equation:

V CO _(2 corrected) =V CO ₂ +FRC×ΔPet _(CO) ₂ /P _(bar),

[0090] where ΔPet_(CO) ₂ is the breath-to-breath change in Pet_(CO) ₂ .

[0091] Baseline Pet_(CO) ₂ and VCO ₂ values, which are also referred toas “before re-breathing Pet_(CO) ₂ ” and “before re-breathing VCO ₂,”respectively, occur during normal breathing and may be calculated as theaverage of a group of samples taken before the re-breathing process(e.g., the average of all samples between about 27 and 0 seconds beforethe start of a known re-breathing process). A VCO ₂ value, which istypically referred to as “during re-breathing VCO ₂,” is calculatedduring the re-breathing process. “During re-breathing VCO ₂” may becalculated as the average VCO ₂ during the interval of 25 to 30 secondsinto the re-breathing period.

[0092] The content of CO₂ in the alveolar blood during the re-breathingprocess may then be calculated by employing a regression line, whichfacilitates prediction of the stable, or unchanging, content of alveolarCO₂. Preferably, PA _(CO) ₂ is plotted against the breath-to-breathchange in content of alveolar CO₂ (ΔCA _(CO) ₂ ). A graph line that isdefined by the plotted points is regressed, and the intersection betweenPA _(CO) ₂ and zero ΔCA _(CO) ₂ is the predicted stable content ofalveolar CO₂.

[0093] Pulmonary capillary blood flow may then be calculated as follows:$Q_{pcbf} = {\frac{\left\lbrack {{{before}\quad {re}\text{-}{breathing}\quad V_{{CO}_{2}}} - {{during}\quad {re}\text{-}{breathing}\quad V_{{CO}_{2}}}} \right\rbrack}{\left\lbrack {{{during}\quad {re}\text{-}{breathing}\quad C_{A_{{CO}_{2}}}} - {{before}\quad {re}\text{-}{breathing}\quad C_{A_{{CO}_{2}}}}} \right\rbrack}.}$

[0094] Operation Logic of a Computer Program for Determining CardiacOutput or Pulmonary Capillary Blood Flow

[0095] The operation logic of an exemplary computer program that directsthe execution of the method of the present invention is brieflyillustrated in the flow diagram of FIG. 11. The computer 76 (see FIG. 3)may be programmed to detect the end of an exhalation, at 200, at whichpoint the computer 76 collects data from the carbon dioxide sensor 74and the flow meter 72 (see FIG. 3) and calculates Pet_(CO) ₂ VCO ₂, thefraction of inspired CO₂, and the airway deadspace values at 202. Thecomputer 76 then calculates FRC, at 204, according to the previouslydescribed equation, and in accordance with the program. The program alsodirects the computer 76 to correct the V_(CO) ₂ value, at 206, inaccordance with the previously described equation. At determinedintervals of time (e.g., two seconds), the CO₂ and VCO ₂ values arere-calculated, at 210, to provide data samples at evenly spaced times,not on the respiratory rate, which may be variable. This technique istypically referred to as “re-sampling” the data.

[0096] The computer 76, in accordance with the program, then calculatesthe estimated partial pressure of CO₂ (P_(CO) ₂ ) in the paralleldeadspace, at 212, and calculates the estimated P_(CO) ₂ in the alveoli,at 214, using the equations described previously. At that point,re-breathing is initiated, at 216, and a deadspace volume is introducedin the re-breathing circuit. Again, the computer 76, in accordance withthe programming thereof, collects data from the carbon dioxide sensor 74and the flow meter 72 (see FIG. 3) and, from that data, determines thechange in VCO ₂ and the change in partial pressure of alveolar CO₂ (PA_(CO) ₂ ) induced by the introduction of the deadspace, at 218. If thecalculated change in VCO ₂ is less than a predetermined minimumpercentage (e.g., 20%) or exceeds a predetermined maximum percentage(e.g., 80%) of the baseline VCO ₂, or if the change in partial pressureof alveolar CO₂ is less than or exceeds predetermined threshold minimumand maximum pressures (e.g., 3 mm Hg or 20 mm Hg), determined at 220,then the operator is notified to accordingly modify the volume of thepartial re-breathing deadspace, at 222. Baseline values may then becanceled, at 224 or 232, then recalculated, as suggested by arrow 226 orarrow 234. Alternatively, the computer 76 may signal mechanical orelectromechanical means associated with the adjustable deadspace toautomatically modify the volume thereof.

[0097] Upon proper adjustment of the adjustable deadspace and therecalculation of baseline Pet_(CO) ₂ , VCO ₂, inspired CO₂ and airwaydeadspace values, the alveolar partial pressure (PA _(CO) ₂ ) isconverted by the software program to CO₂ content of the alveolar(pulmonary) capillaries (CA _(CO) ₂ or Cc′_(CO) ₂ ). The change in theCO₂ content of the alveolar blood induced by having the patientre-breathe a volume of previously exhaled gases from the deadspace isthen calculated, at 236. From these values, the cardiac output orpulmonary capillary blood flow of the patient may be calculated, at 238,in accordance with the previously described equation or otherwise, asknown in the art.

[0098] Although the foregoing description contains many specifics, theseshould not be construed as limiting the scope of the present invention,but merely as providing illustrations of some of the presently preferredembodiments. Similarly, other embodiments of the invention may bedevised which do not depart from the spirit or scope of the presentinvention. The scope of the invention is, therefore, indicated andlimited only by the appended claims and their legal equivalents, ratherthan by the foregoing description. All additions, deletions andmodifications to the invention as disclosed herein which fall within themeaning and scope of the claims are to be embraced thereby within theirscope.

What is claimed is:
 1. An apparatus for introducing deadspace into abreathing circuit, comprising: a primary expiratory pathway through thebreathing circuit; a deadspace portion of the breathing circuit locatedto receive gases exhaled by a patient upon positioning the breathingcircuit in communication with an airway of the patient, said deadspaceportion communicating with said primary expiratory pathway at least onejunction thereof with said primary expiratory pathway; a flow restrictorpositioned along said primary expiratory pathway, downstream from saidat least one junction; and a two-way valve located and positionable soas to, along with said flow restrictor, prevent gases from flowing intosaid deadspace portion and allow gases to flow into said deadspaceportion.
 2. The apparatus of claim 1, wherein said deadspace portioncomprises at least a volume adjustable section.
 3. The apparatus ofclaim 2, wherein said volume adjustable section is length expandable andlength contractible.
 4. A method for estimating the partial pressure ofcarbon dioxide in the alveolar blood (PA _(CO) ₂ ) of an individual,comprising: calculating a concentration of carbon dioxide in a paralleldeadspace (PDS_(CO) ₂ ) of an airway of the individual on abreath-by-breath basis; and determining a partial pressure of end tidalcarbon dioxide (PetCO₂) of the individual.
 5. The method of claim 4,further comprising determining a perfusion ratio (r).
 6. The method ofclaim 5, wherein: P A _(CO) ₂ =[PetCO ₂−(1−r)×PDS_(CO) ₂ ]/r.
 7. Themethod of claim 4, wherein said calculating comprises: determining amixed inspired volume of carbon dioxide (ViCO₂) inhaled by theindividual; at least estimating an airway deadspace of the individual;determining a partial pressure of end tidal carbon dioxide (PetCO₂) of aprevious breath of the individual; and determining a tidal volume(V_(t)) of the individual's breathing.
 8. The method of claim 7, whereinsaid calculating further comprises: at least estimating a functionalresidual capacity (FRC) of alveoli of the individual's lungs.
 9. Themethod of claim 8, wherein $\begin{matrix}{{{PDS}_{{CO}_{2}}(n)} = {\left\{ {\left\lbrack {{FRC}/\left( {{FRC} + V_{t}} \right)} \right\rbrack \times {{PDS}_{{CO}_{2}}\left( {n - 1} \right)}} \right\} +}} \\{\left( {\left\{ {\left\lbrack {{{Vi}{CO}}_{2} + \left( {{deadspace} \times {{{Pet}{CO}}_{2}\left( {n - 1} \right)}} \right)} \right\rbrack/V_{t}} \right\} \times} \right.} \\{\left. \left\lbrack {V_{t}/\left( {V_{t} + {FRC}} \right)} \right\rbrack \right),}\end{matrix}$

where (n) indicates a parameter for a current breath and (n−1)represents a parameter for an immediately preceding breath.
 10. A methodfor estimating the cardiac output of an individual, comprising:determining a carbon dioxide elimination (VCO ₂) of the individual for abefore re-breathing period and for a during re-breathing period;calculating a carbon dioxide elimination difference between said carbondioxide elimination of said before re-breathing period and said carbondioxide elimination of said during re-breathing period; estimating apartial pressure of carbon dioxide in alveolar blood (PACO₂) of theindividual for said before re-breathing period and for said duringre-breathing period respectively based on partial pressure of end tidalcarbon dioxide (PetCO₂) measurements of the individual during saidbefore re-breathing period and during said during re-breathing period;converting each estimation of said partial pressure of carbon dioxide inalveolar blood to a carbon dioxide content (C_(CO) ₂ ); calculating acarbon dioxide content difference between said carbon dioxide content ofsaid before re-breathing period and said carbon dioxide content of saidduring re-breathing period; and dividing said carbon dioxide eliminationdifference by said carbon dioxide content difference.